Microsporidia are obligate intracellular protozoan parasites that have many similarities to fungi. Two species of this parasite have been found to infect the gastrointestinal tract of AIDS patients and other immunocompromized individuals. The most commonly encountered microsporidia, Enterocytozoon bieneusi, has not been reliably cultured and is resistant to the clinical drug of choice, albendazole, while Encephalitozoon intestinalis can be cultured in several cell lines and clinical infections are usually cleared by albendazole treatment. The microsporidia Vittaforma corneae is resistant to albendazole, but can be cultured, and has been used as a surrogate for E. bieneusi. Most in vitro microsporidia infection studies have been performed using cell lines chosen because they were easily infected or were robust and therefore appropriate for use in drug screening. Polarized intestinal epithelial cell line models would be expected to more accurately reflect the human infection in terms of host-parasite relationships, epithelial physiology and inflammatory response to infection. The purpose of this R21 application is to develop polarized intestinal epithelial models of microsporidiosis that will be characterized on the basis of the effects of infection with E. intestinalis, the E. bieneusi surrogate, V. corneae and Brachiola (Nosema) algerae on epithelial monolayer permeability and integrity, differentiation and ultrastructure, and response to apoptotic signals. These models will be used to asses the role of host cell phagocytosis in infection, the cytokine and chemokine responses to infection, the effect/role of host cell multidrug resistance transporters in infection, and the epithelial repair following infection chemotherapeutic treatment. The development and characterization of polarized intestinal epithelial microsporidiosis models will further the understanding of in situ host-parasite relationships for these three parasite species, while comparing E. intestinatis and V. corneae infections in these models may contribute to understanding the unique features of the E. bieneusi infections.